Specs: i5-7400 3.00 GHz, 16GB RAM, 1060 6GB, Crucial 500MX SSD 500GB + Kingston A400 SSD 240GB.
Hitfilm Express: Version 16
Preview is laggy. All I add, sometimes, is a "Fade to Colour" and it will lag over that part of the video and skip to where it is fully presented.
I have Hitfilm Express on "High Priority" in Task Manager
I rarely have other applications open, sometimes Spotify.
I have de-fragged in the last 1-2 weeks.
I have not overclocked my CPU due to lack of fans (only 2 slots on motherboard, working on getting an adapter)
I'm in the process of clearing up space within my Drives and moving less useful stuff to my hard drive.